Embodiments described herein relate generally to exhaust gas recirculation (EGR) systems in vehicles. More specifically, embodiments described herein relate to condensate removal from EGR systems in vehicles.
Exhaust gas recirculation (EGR) is used to reduce nitrogen oxide (NOx) emissions in both gasoline and diesel engines. NOx is primarily formed when a mix of nitrogen and oxygen is subjected to high temperatures. EGR systems recirculate a portion of an engine's exhaust gas back to the engine cylinders. Intermixing fresh, incoming air with recirculated exhaust gas dilutes the mix, which lowers the flame temperature and reduces the amount of excess oxygen. The exhaust gas also increases the specific heat capacity of the mix, which lowers the peak combustion temperature. Since NOx is more readily formed at high temperatures, the EGR system limits the generation of NOx by keeping the temperatures low.
Most EGR systems include at least one EGR valve and optionally at least one EGR cooler connected in series between an exhaust manifold and an intake manifold of an engine. Some engines, especially compression ignition or diesel engines, use coolers that cool the portion of exhaust gas being recirculated. The cooled exhaust gas has a lower latent heat content and can aid in lowering combustion temperatures even further. In general, engines using EGR to lower their NOx emissions can attain lower emissions by cooling the recirculated exhaust gas as much as possible.
Exhaust gas constituents in the exhaust gas being recirculated to the intake manifold may present problems when the exhaust gas is cooled below a condensation temperature of those constituents. Various hydrocarbons may condense onto engine components and may present issues such as sluggish performance or even sticking of moving parts. These issues are especially evident when an engine starts under cold ambient conditions, when most engine components are cold and exhaust gas constituents condense more readily onto the engine components.
Engines in the past have attempted to cope with the problem of condensation of exhaust gas constituents by delaying initiation of EGR under cold start conditions, limiting the amount of exhaust gas being recirculated, or limiting the amount of cooling applied to the recirculated exhaust gas in an effort to minimize the degree and amount of condensates. These measures, although effective in increasing the service life of engine components and decreasing the likelihood of failures, may be insufficient in addressing the impact they have on the emissions generated by the engine. The more delayed the initiation of EGR becomes, or, the limited amount of cooling of the exhaust gas, qualitatively increases the emissions generated by the engine.
Some engine designs cope with the issue of condensation by placing the EGR valve upstream, or on the “hot side” of the EGR cooler. This placement of the EGR valve ensures that the valve will not be exposed to cooled exhaust gas, and thus be immune to the condensation effects that result from the cooling. These configurations may expose the EGR valve to high temperatures which may reduce the service life of the valve or require higher cost and/or complexity of the valve.